Laser produced plasma EUV light source

ABSTRACT

Methods and apparatus for producing EUV from plasma are disclosed. The apparatus includes a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region. The plasma produces EUV radiation, wherein the droplet source comprises a nozzle having an orifice configured for ejecting a fluid and a sub-system having an electro-actuable element producing a disturbance in the fluid to cause at least some of the droplets to coalesce prior to being irradiated. The electro-actuable element is coupled to nozzle using an adhesive that has a high modulus at the nozzle operating temperature. Improvements also include tuning the nozzle assembly to more closely match the modulation waveform frequency with one of the resonance frequencies of the nozzle assembly by optimizing one of a mass, a shape, or material composition of at least one component in the nozzle assembly.

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 12/721,317, filed on Mar. 10, 2010 now U.S. Pat.No. 8,158,960, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, andwhich is a continuation-in-part application of U.S. patent applicationSer. No. 11/827,803, filed on Jul. 13, 2007, issued on Mar. 1, 2011 asU.S. Pat. No. 7,897,947, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCEHAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, theentire contents of the above application(s) is/are hereby incorporatedby reference herein.

The present application is related to U.S. patent application Ser. No.11/358,988 filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMA EUVLIGHT SOURCE WITH PRE-PULSE, U.S. patent application Ser. No. 11/067,124filed on Feb. 25, 2005, entitled METHOD AND APPARATUS FOR EUV PLASMASOURCE TARGET DELIVERY, U.S. patent application Ser. No. 11/174,443filed on Jun. 29, 2005, entitled LPP EUV PLASMA SOURCE MATERIAL TARGETDELIVERY SYSTEM, U.S. SOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE,U.S. patent application Ser. No. 11/358,992 filed on Feb. 21, 2006,entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, U.S. patent applicationSer. No. 11/174,299 filed on Jun. 29, 2005, and entitled, LPP EUV LIGHTSOURCE DRIVE LASER SYSTEM, U.S. patent application Ser. No. 11/406,216filed on Apr. 17, 2006, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE,U.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006,entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, and U.S.patent application Ser. No. 11/644,153 filed on Dec. 22, 2006, entitled,LASER PRODUCED PLASMA EUV LIGHT SOURCE, U.S. patent application Ser. No.11/505,177 filed on Aug. 16, 2006, entitled EUV OPTICS, U.S. patentapplication Ser. No. 11/452,558 filed on Jun. 14, 2006, entitled DRIVELASER FOR EUV LIGHT SOURCE, U.S. Pat. No. 6,928,093, issued to Webb, etal., on Aug. 9, 2005, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER,U.S. application Ser. No. 11/394,512, filed on Mar. 31, 2006, entitledCONFOCAL PULSE STRETCHER; U.S. application Ser. No. 11/138,001 filed onMay 26, 2005, entitled SYSTEMS AND METHODS FOR IMPLEMENTING ANINTERACTION BETWEEN A LASER SHAPED AS A LINE BEAM AND A FILM DEPOSITEDON A SUBSTRATE; and U.S. application Ser. No. 10/141,216, filed on May7, 2002, now U.S. Pat. No. 6,693,939, entitled, LASER LITHOGRAPHY LIGHTSOURCE WITH BEAM DELIVERY; U.S. Pat. No. 6,625,191 issued to Knowles, etal., on Sep. 23, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REPRATE GAS DISCHARGE LASER SYSTEM; U.S. application Ser. No. 10/012,002,U.S. Pat. No. 6,549,551 issued to Ness, et al., on Apr. 15, 2003,entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL, U.S.application Ser. No. 09/848,043, and U.S. Pat. No. 6,567,450 issued toMyers, et al., on May 20, 2003, entitled VERY NARROW BAND, TWO CHAMBER,HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No.09/943,343, U.S. patent application Ser. No. 11/509,925 filed on Aug.25, 2006, entitled SOURCE MATERIAL COLLECTION UNIT FOR A LASER PRODUCEDPLASMA EUV LIGHT SOURCE, the entire contents of each of which are herebyincorporated by reference herein.

FIELD

The present disclosure relates to extreme ultraviolet (“EUV”) lightsources that provide EUV light from a plasma that is created from atarget material and collected and directed to an intermediate region forutilization outside of the EUV light source chamber, e.g., by alithography scanner/stepper.

BACKGROUND

Extreme ultraviolet light, e.g., electromagnetic radiation havingwavelengths of around 50 nm or less (also sometimes referred to as softx-rays), and including light at a wavelength of about 13.5 nm, can beused in photolithography processes to produce extremely small featuresin substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has at least oneelement, e.g., xenon, lithium or tin, with one or more emission line inthe EUV range. In one such method, often termed laser produced plasma(“LPP”) the required plasma can be produced by irradiating a targetmaterial having the required line-emitting element, with a laser beam.

One particular LPP technique involves irradiating a target materialdroplet with one or more pre-pulse(s) followed by a main pulse. In thisregard, CO₂ lasers may present certain advantages as a drive laserproducing “main” pulses in an LPP process. This may be especially truefor certain target materials such as molten tin droplets. For example,one advantage may include the ability to produce a relatively highconversion efficiency e.g., the ratio of output EUV in-band power todrive laser input power.

In more theoretical terms, LPP light sources generate EUV radiation bydepositing laser energy into a source element, such as xenon (Xe), tin(Sn) or lithium (Li), creating a highly ionized plasma with electrontemperatures of several 10's of eV. The energetic radiation generatedduring de-excitation and recombination of these ions is emitted from theplasma in all directions. In one common arrangement, anear-normal-incidence mirror (often termed a “collector mirror”) ispositioned at a distance from the plasma to collect, direct (and in somearrangements, focus) the light to an intermediate location, e.g., focalpoint. The collected light may then be relayed from the intermediatelocation to a set of scanner optics and ultimately to a wafer. In morequantitative terms, one arrangement that is currently being developedwith the goal of producing about 100 W at the intermediate locationcontemplates the use of a pulsed, focused 10-12 kW CO₂ drive laser whichis synchronized with a droplet generator to sequentially irradiate about40,000-100,000 tin droplets per second. For this purpose, there is aneed to produce a stable stream of droplets at a relatively highrepetition rate (e.g., 40-100 kHz or more) and deliver the droplets toan irradiation site with high accuracy and good repeatability in termsof timing and position (i.e. with very small “jitter”) over relativelylong periods of time.

For a typical LPP setup, target material droplets are generated and thentravel within a vacuum chamber to an irradiation site where they areirradiated, e.g. by a focused laser beam. In addition to generating EUVradiation, these plasma processes also typically generate undesirableby-products in the plasma chamber (e.g., debris) that can potentiallydamage or reduce the operational efficiency of the various plasmachamber optical elements. These debris can include high-energy ions andscattered debris from the plasma formation, e.g., atoms and/orclumps/microdroplets of source material. For this reason, it is oftendesirable to use so-called “mass limited” droplets of source material toreduce or eliminate the formation of debris. The use of “mass limited”droplets may also result in a reduction in source material consumption.Techniques to achieve a mass-limited droplet may involve diluting thesource material and/or using relatively small droplets. For example, theuse of droplets as small as 10-50 μm is currently contemplated.

In addition to their effect on optical elements in the vacuum chamber,the plasma by-products may also adversely affect the droplet(s)approaching the irradiation site (i.e., subsequent droplets in thedroplet stream). In some cases, interactions between droplets and theplasma by-products may result in a lower EUV output for these droplets.In this regard, U.S. Pat. No. 6,855,943 (hereinafter the '943 patent)which issued to Shields on Feb. 15, 2005, and is entitled “DROPLETTARGET DELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREMEULTRAVIOLET LIGHT SOURCE” discloses a technique in which only some ofthe droplets in a droplet stream, e.g., every third droplet, isirradiated to produce a pulsed EUV light output. As disclosed in the'943 patent, the nonparticipating droplets (so-called buffer droplets)advantageously shield the next participating droplet from the effects ofthe plasma generated at the irradiation site. However, the use of bufferdroplets may increase source material consumption and/or vacuum chambercontamination and/or may require droplet generation at a frequency muchhigher (e.g., by a factor of two or more) than required without the useof buffer droplets. On the other hand, if the spacing between dropletscan be increased, the use of buffer droplets may be reduced oreliminated. Thus, droplet size, spacing and timing consistency (i.e.,jitter) are among the factors to be considered when designing a dropletgenerator for an LPP EUV light source.

One technique for generating droplets involves melting a targetmaterial, e.g., tin, and then forcing it under high pressure through arelative small diameter orifice, e.g, 0.5-30 μm. Under most conditions,naturally occurring instabilities, e.g. noise, in the stream exiting theorifice may cause the stream to break-up into droplets. In order tosynchronize the droplets with optical pulses of the LPP drive laser, arepetitive disturbance with an amplitude exceeding that of the randomnoise may be applied to the continuous stream. By applying a disturbanceat the same frequency (or its higher harmonics) as the repetition rateof the pulsed laser, the droplets can be synchronized with the laserpulses. In the past, the disturbance has typically been applied to thestream by driving an electro-actuatable element (such as a piezoelectricmaterial) with a waveform of a single frequency such as a sinusoidalwaveform or its equivalent.

As used herein, the term “electro-actuatable element” and itsderivatives, means a material or structure which undergoes a dimensionalchange when subjected to a voltage, electric field, magnetic field, orcombinations thereof and includes, but is not limited to, piezoelectricmaterials, electrostrictive materials and magnetostrictive materials.

In general, for the application of single frequency, non-modulatedwaveform disturbances such as a sinusoidal waveform, the spacing betweendroplets increases as the disturbance frequency decreases (i.e., holdingother factors such as pressure and orifice diameter constant). However,as disclosed in “Drop Formation From A Vibrating Orifice GeneratorDriven By Modulated Electrical Signals” (G. Brenn and U. Lackermeler,Phys. Fluids 9, 3658 (1997), the contents of which are incorporated byreference herein), for disturbance frequencies below about 0.3ν/(πd),where ν is the stream velocity and d is the diameter of the continuousliquid stream, more than one droplet may be generated for eachdisturbance period. Thus, for a 10 μm liquid jet at a stream velocity ofabout 50 m/s, the calculated minimum frequency below which more than onedroplet per period may be produced is about 480 kHz (note: it iscurrently envisioned that a droplet repetition rate of 40-100 kHz andvelocities of about 30-100 m/s may be desirable for LPP EUV processes).The net result is that for the application of single frequency,non-modulated waveform disturbances, the spacing between droplets isfundamentally limited and cannot exceed approximately 3.33πd. Asindicated above, it may be desirable to supply a sufficient distancebetween adjacent droplets in the droplet stream to reduce/eliminate theeffect of the debris from the plasma on approaching droplet(s).Moreover, because the limitation on spacing is proportional to streamdiameter, and as a consequence droplet size, this limitation can beparticularly severe in applications such as LPP EUV light sources whererelatively small, mass-limited, droplets are desirable (see discussionabove).

With the above in mind, Applicants disclose a laser produced plasma, EUVlight source having a droplet stream produced using a modulateddisturbance waveform, and corresponding methods of use.

SUMMARY

The invention relates, in an embodiment, to a plasma generating systemcomprising a source of target material droplets and a laser producing abeam irradiating the droplets at an irradiation region. The plasmaproduces EUV radiation, wherein the droplet source comprises a nozzlehaving an orifice configured for ejecting a fluid and a sub-systemhaving an electro-actuable element producing a disturbance in the fluidto cause at least some of the droplets to coalesce prior to beingirradiated. The electro-actuable element is coupled to nozzle using anadhesive that has a high modulus at the nozzle operating temperature. Inone or more embodiments, the adhesive is polyimide-based orbismaleimide-based.

In yet another embodiment, the invention relates to a method for tuningthe frequency response of the nozzle assembly of a plasma generatingsystem. The nozzle assembly is configured for ejecting target materialdroplets. The plasma generating system comprises the nozzle assembly anda laser producing a beam irradiating the droplets at an irradiationregion. The plasma produces EUV radiation, wherein the nozzle assemblycomprises a nozzle having an orifice configured for ejecting a fluid anda sub-system having an electro-actuable element producing a modulationwaveform having a modulation frequency to cause disturbance in the fluidto cause at least some of the droplets to coalesce prior to beingirradiated. The method includes ascertaining the frequency response ofthe nozzle assembly, including at least one resonance frequency. Themethod also includes modifying the nozzle assembly to cause the at leastone resonance frequency to more closely match with the modulationfrequency, wherein the modifying including optimizing one of a mass, ashape, or material composition of at least one component in the nozzleassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified, schematic view of a laser produced plasma EUVlight source;

FIG. 2 shows a schematic a simplified droplet source;

FIGS. 2A-2D illustrate several different techniques for coupling anelectro-actuable element with a fluid to create a disturbance in astream exiting an orifice;

FIG. 3 (Prior Art) illustrates the pattern of droplets resulting from asingle frequency, non-modulated disturbance waveform;

FIG. 4 illustrates the pattern of droplets resulting from an amplitudemodulated disturbance waveform;

FIG. 5 illustrates the pattern of droplets resulting from a frequencymodulated disturbance waveform;

FIG. 6 shows photographs of tin droplets obtained for a singlefrequency, non-modulated waveform disturbance and several frequencymodulated waveform disturbances;

FIG. 7 illustrates a droplet pattern achievable using a modulatedwaveform disturbance in which droplet pairs reach the irradiation regionallowing one droplet to shield subsequent droplet pairs from plasmadebris;

FIG. 8 illustrates a droplet pattern achievable using a modulatedwaveform disturbance in which droplet pairs reach the irradiation regionwith a first droplet reflecting light into a self-directing laser systemto initiate a discharge which irradiates the second droplet to producean EUV emitting plasma;

FIG. 8A illustrates a droplet pattern in which droplet doublets reachthe irradiation site with each droplet in the droplet doublet beingirradiated to produce plasma;

FIG. 9 shows a representation of a square wave as a superposition of oddharmonics of a sine wave signal;

FIG. 10 shows images of droplets obtained with a square wave modulationat 30 kHz taken at ˜40 mm from the output orifice;

FIG. 11 shows images of droplets obtained with a square wave modulationat 30 kHz taken at ˜120 mm from the output orifice;

FIGS. 12A-D show experimental results for a rectangular wave (FIG. 12A)modulation including a frequency spectrum (FIG. 12B) for a rectangularwave; an image of droplets taken at 20 mm from the output orifice (FIG.12C) and an image of coalesced droplets taken at 450 mm from the outputorifice (FIG. 12D);

FIGS. 13A-D show experimental results for fast pulses (FIG. 13A)modulation including a frequency spectrum (FIG. 13B) for a fast pulse;an image of droplets taken at 20 mm from the output orifice (FIG. 13C)and an image of coalesced droplets taken at 450 mm from the outputorifice (FIG. 13D);

FIGS. 14A-D show experimental results for fast ramp wave (FIG. 14A)modulation including a frequency spectrum (FIG. 14B) for a fast rampwave; an image of droplets taken at 20 mm from the output orifice (FIG.14C) and an image of coalesced droplets taken at 450 mm from the outputorifice (FIG. 14D); and

FIGS. 15A-D show experimental results for a sine function wave (FIG.15A) modulation including a frequency spectrum (FIG. 15B) for a sinefunction wave; an image of droplets taken at 20 mm from the outputorifice (FIG. 15C) and an image of coalesced droplets taken at 450 mmfrom the output orifice (FIG. 15D).

FIG. 16 shows a plot of the tensile modulus (Young's Modulus) of atypical epoxy with respect to temperature.

FIG. 17 shows a plot of a nozzle assembly frequency response, includingan amplitude plot line and a phase shift plot line versus frequency.

FIGS. 18A and 18B show, in accordance with one or more embodiments ofthe invention, an implementation wherein a multi-piece electro-actuableelement is coupled to the nozzle without using adhesive.

DETAILED DESCRIPTION

With initial reference to FIG. 1, there is shown a schematic view of anEUV light source, e.g., a laser-produced-plasma, EUV light source 20according to one aspect of an embodiment. As shown in FIG. 1, anddescribed in further detail below, the LPP light source 20 may include asystem 22 for generating a train of light pulses and delivering thelight pulses into a chamber 26. As detailed below, each light pulse maytravel along a beam path from the system 22 and into the chamber 26 toilluminate a respective target droplet at an irradiation region 28.

Suitable lasers for use as the system 22 shown in FIG. 1 may include apulsed laser device, e.g., a pulsed gas discharge CO₂ laser deviceproducing radiation at 9.3 μm or 10.6 μm, e.g., with DC or RFexcitation, operating at relatively high power, e.g., 10 kW or higherand high pulse repetition rate, e.g., 50 kHz or more. In one particularimplementation, the laser may be an axial-flow RF-pumped CO₂ laserhaving a MOPA configuration with multiple stages of amplification andhaving a seed pulse that is initiated by a Q-switched Master Oscillator(MO) with low energy and high repetition rate, e.g., capable of 100 kHzoperation. From the MO, the laser pulse may then be amplified, shaped,and/or focused before entering the LPP chamber. Continuously pumped CO₂amplifiers may be used for the system 22. For example, a suitable CO₂laser device having an oscillator and three amplifiers (O-PA1-PA2-PA3configuration) is disclosed in U.S. patent application Ser. No.11/174,299 filed on Jun. 29, 2005, entitled, LPP EUV LIGHT SOURCE DRIVELASER SYSTEM, the entire contents of which have been previouslyincorporated by reference herein. Alternatively, the laser may beconfigured as a so-called “self-targeting” laser system in which thedroplet serves as one mirror of the optical cavity. In some“self-targeting” arrangements, a master oscillator may not be required.Self-targeting laser systems are disclosed and claimed in U.S. patentapplication Ser. No. 11/580,414, filed on Oct. 13, 2006 entitled, DRIVELASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, the entire contents ofwhich have been previously incorporated by reference herein.

Depending on the application, other types of lasers may also besuitable, e.g., an excimer or molecular fluorine laser operating at highpower and high pulse repetition rate. Examples include, a solid statelaser, e.g., having a fiber or disk shaped active media, an excimerlaser having one or more chambers, e.g., an oscillator chamber and oneor more amplifying chambers (with the amplifying chambers in parallel orin series), a master oscillator/power oscillator (MOPO) arrangement, apower oscillator/power amplifier (POPA) arrangement, or a solid statelaser that seeds one or more excimer or molecular fluorine amplifier oroscillator chambers, may be suitable. Other designs are possible.

As further shown in FIG. 1, the EUV light source 20 may also include atarget material delivery system 24, e.g., delivering droplets of atarget material into the interior of a chamber 26 to the irradiationregion 28 where the droplets will interact with one or more lightpulses, e.g., zero, one or more pre-pulses and thereafter one or moremain pulses, to ultimately produce a plasma and generate an EUVemission. The target material may include, but is not necessarilylimited to, a material that includes tin, lithium, xenon or combinationsthereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., maybe in the form of liquid droplets and/or solid particles containedwithin liquid droplets. For example, the element tin may be used as puretin, as a tin compound, e.g., SnBr₄, SnBr₂, SnH₄, as a tin alloy, e.g.,tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or acombination thereof. Depending on the material used, the target materialmay be presented to the irradiation region 28 at various temperaturesincluding room temperature or near room temperature (e.g., tin alloys,SnBr₄) at an elevated temperature, (e.g., pure tin) or at temperaturesbelow room temperature, (e.g., SnH₄), and in some cases, can berelatively volatile, e.g., SnBr₄. More details concerning the use ofthese materials in an LPP EUV source is provided in U.S. patentapplication Ser. No. 11/406,216, filed on Apr. 17, 2006, entitledALTERNATIVE FUELS FOR EUV LIGHT SOURCE, the contents of which have beenpreviously incorporated by reference herein.

Continuing with FIG. 1, the EUV light source 20 may also include anoptic 30, e.g., a collector mirror in the form of a truncated ellipsoidhaving, e.g., a graded multi-layer coating with alternating layers ofMolybdenum and Silicon. FIG. 1 shows that the optic 30 may be formedwith an aperture to allow the light pulses generated by the system 22 topass through and reach the irradiation region 28. As shown, the optic 30may be, e.g., an ellipsoidal mirror that has a first focus within ornear the irradiation region 28, and a second focus at a so-calledintermediate region 40, where the EUV light may be output from the EUVlight source 20 and input to a device utilizing EUV light, e.g., anintegrated circuit lithography tool (not shown). It is to be appreciatedthat other optics may be used in place of the ellipsoidal mirror forcollecting and directing light to an intermediate location forsubsequent delivery to a device utilizing EUV light, for example, theoptic may be parabolic or may be configured to deliver a beam having aring-shaped cross-section to an intermediate location, see e.g. U.S.patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, entitledEUV OPTICS, the contents of which are hereby incorporated by reference.

Continuing with reference to FIG. 1, the EUV light source 20 may alsoinclude an EUV controller 60, which may also include a firing controlsystem 65 for triggering one or more lamps and/or laser devices in thesystem 22 to thereby generate light pulses for delivery into the chamber26. The EUV light source 20 may also include a droplet positiondetection system which may include one or more droplet imagers 70 thatprovide an output indicative of the position of one or more droplets,e.g., relative to the irradiation region 28. The imager(s) 70 mayprovide this output to a droplet position detection feedback system 62,which can, e.g., compute a droplet position and trajectory, from which adroplet error can be computed, e.g., on a droplet-by-droplet basis, oron average. The droplet error may then be provided as an input to thecontroller 60, which can, for example, provide a position, directionand/or timing correction signal to the system 22 to control a sourcetiming circuit and/or to control a beam position and shaping system,e.g., to change the location and/or focal power of the light pulsesbeing delivered to the irradiation region 28 in the chamber 26.

The EUV light source 20 may include one or more EUV metrologyinstruments for measuring various properties of the EUV light generatedby the source 20. These properties may include, for example, intensity(e.g., total intensity or intensity within a particular spectral band),spectral bandwidth, polarization, beam position, pointing, etc. For theEUV light source 20, the instrument(s) may be configured to operatewhile the downstream tool, e.g., photolithography scanner, is on-line,e.g., by sampling a portion of the EUV output, e.g., using a pickoffminor or sampling “uncollected” EUV light, and/or may operate while thedownstream tool, e.g., photolithography scanner, is off-line, forexample, by measuring the entire EUV output of the EUV light source 20.

As further shown in FIG. 1, the EUV light source 20 may include adroplet control system 90, operable in response to a signal (which, insome implementations may include the droplet error described above, orsome quantity derived therefrom) from the controller 60, to e.g., modifythe release point of the target material from a droplet source 92 and/ormodify droplet formation timing, to correct for errors in the dropletsarriving at the desired irradiation region 28 and/or synchronize thegeneration of droplets with the pulsed laser system 22.

FIG. 2 illustrates the components of a simplified droplet source 92 inschematic format. As shown there, the droplet source 92 may include areservoir 94 holding a fluid, e.g. molten tin, under pressure. Alsoshown, the reservoir 94 may be formed with an orifice 98 allowing thepressurized fluid 96 to flow through the orifice establishing acontinuous stream 100 which subsequently breaks into a plurality ofdroplets 102 a, b.

Continuing with FIG. 2, the droplet source 92 shown further includes asub-system producing a disturbance in the fluid having anelectro-actuatable element 104 that is operably coupled with the fluid96 and a signal generator 106 driving the electro-actuatable element104. FIGS. 2A-2D show various ways in which one or moreelectro-actuatable elements may be operably coupled with the fluid tocreate droplets. Beginning with FIG. 2A, an arrangement is shown inwhich the fluid is forced to flow from a reservoir 108 under pressurethrough a tube 110, e.g., capillary tube, having an inside diameterbetween about 0.5-0.8 mm, and a length of about 10 to 50 mm, creating acontinuous stream 112 exiting an orifice 114 of the tube 110 whichsubsequently breaks up into droplets 116 a,b. As shown, anelectro-actuatable element 118 may be coupled to the tube. For example,an electro-actuatable element may be coupled to the tube 110 to deflectthe tube 110 and disturb the stream 112. FIG. 2B shows a similararrangement having a reservoir 120, tube 122 and a pair ofelectro-actuatable elements 124, 126, each coupled to the tube 122 todeflect the tube 122 at a respective frequency. FIG. 2C shows anothervariation in which a plate 128 is positioned in a reservoir 130 moveableto force fluid through an orifice 132 to create a stream 134 whichbreaks into droplets 136 a,b. As shown, a force may be applied to theplate 128 and one or more electro-actuatable elements 138 may be coupledto the plate to disturb the stream 134. It is to be appreciated that acapillary tube may be used with the embodiment shown in FIG. 2C. FIG. 2Dshows another variation, in which a fluid is forced to flow from areservoir 140 under pressure through a tube 142 creating a continuousstream 144, exiting an orifice 146 of the tube 142, which subsequentlybreaks-up into droplets 148 a,b. As shown, an electro-actuatable element150, e.g., having a ring-like shape, may be positioned around the tube142. When driven, the electro-actuatable element 142 may selectivelysqueeze and/or un-squeeze the tube 142 to disturb the stream 144. It isto be appreciated that two or more electro-actuatable elements may beemployed to selectively squeeze the tube 142 at respective frequencies.

More details regarding various droplet dispenser configurations andtheir relative advantages may be found in U.S. patent application Ser.No. 11/358,988, filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMAEUV LIGHT SOURCE WITH PRE-PULSE, U.S. patent application Ser. No.11/067,124 filed on Feb. 25, 2005, entitled METHOD AND APPARATUS FOR EUVPLASMA SOURCE TARGET DELIVERY, and U.S. patent application Ser. No.11/174,443 filed on Jun. 29, 2005, entitled LPP EUV PLASMA SOURCEMATERIAL TARGET DELIVERY SYSTEM, the contents of each of which arehereby incorporated by reference.

FIG. 3 (Prior Art) illustrates the pattern of droplets 200 resultingfrom a single frequency, sine wave disturbance waveform 202 (fordisturbance frequencies above about 0.3ν/(πd). It can be seen that eachperiod of the disturbance waveform produces a droplet. FIG. 3 alsoillustrates that the droplets do not coalesce together, but rather, eachdroplet is established with the same initial velocity.

FIG. 4 illustrates the pattern of droplets 300 initially resulting froman amplitude modulated disturbance waveform 302, which however is unlikethe disturbance waveform 202 described above, in that it is not limitedto disturbance frequencies above about 0.3ν/(πd)). It can be seen thatthe amplitude modulated waveform disturbance 302 includes twocharacteristic frequencies, a relatively large frequency, e.g., carrierfrequency, corresponding to wavelength λ_(c), and a smaller frequency,e.g., modulation frequency, corresponding to wavelength, λ_(m). For thespecific disturbance waveform example shown in FIG. 4, the modulationfrequency is a carrier frequency subharmonic, and in particular, themodulation frequency is a third of the carrier frequency. With thiswaveform, FIG. 4 illustrates that each period of the disturbancewaveform corresponding to the carrier wavelength, λ_(c) produces adroplet. FIG. 4 also illustrates that the droplets coalesce together,resulting in a stream of larger droplets 304, with one larger dropletfor each period of the disturbance waveform corresponding to themodulation wavelength, λ_(m). Arrows 306 a,b show the initial relativevelocity components that are imparted on the droplets by the modulatedwaveform disturbance 302, and are responsible for the dropletcoalescence.

FIG. 5 illustrates the pattern of droplets 400 initially resulting froma frequency modulated disturbance waveform 402, which, like thedisturbance waveform 302 described above, is not limited to disturbancefrequencies above about 0.3ν/(πd). It can be seen that the frequencymodulated waveform disturbance 402 includes two characteristicfrequencies, a relatively large frequency, e.g. carrier frequency,corresponding to wavelength λ_(c), and a smaller frequency, e.g.modulation frequency, corresponding to wavelength, λ_(m). For thespecific disturbance waveform example shown in FIG. 5, the modulationfrequency is a carrier frequency harmonic, and in particular, themodulation frequency is a third of the carrier frequency. With thiswaveform, FIG. 5 illustrates that each period of the disturbancewaveform corresponding to the carrier wavelength, λ_(c) produces adroplet. FIG. 5 also illustrates that the droplets coalesce together,resulting in a stream of larger droplets 404, with one larger dropletfor each period of the disturbance waveform corresponding to themodulation wavelength, λ_(m). Like the amplitude modulated disturbance(i.e., FIG. 4), initial relative velocity components are imparted on thedroplets by the frequency modulated waveform disturbance 402, and areresponsible for the droplet coalescence.

Although FIGS. 4 and 5 show and discuss embodiments having twocharacteristic frequencies, with FIG. 4 illustrating an amplitudemodulated disturbance having two characteristic frequencies, and FIG. 5illustrating a frequency modulated disturbance having two frequencies,it is to be appreciated that more than two characteristic frequenciesmay be employed and that the modulation may be either angular modulation(i.e., frequency or phase modulation), amplitude modulation, orcombinations thereof.

FIG. 6 shows photographs of tin droplets obtained using an apparatussimilar to FIG. 2D with an orifice diameter of about 70 μm, streamvelocity of ˜30 m/s, for a single frequency, non-modulated waveformdisturbance having a frequency of 100 kHz (top photo); a frequencymodulated waveform disturbance having a carrier frequency of 100 kHz anda modulating frequency of 10 kHz of a relatively strong modulation depth(second from top photo); a frequency modulated waveform disturbancehaving a carrier frequency of 100 kHz and a modulating frequency of 10kHz of a relatively weak modulation depth (third from top photo); afrequency modulated waveform disturbance having a carrier frequency of100 kHz and a modulating frequency of 15 kHz (fourth from top photo), afrequency modulated waveform disturbance having a carrier frequency of100 kHz and a modulating frequency of 20 kHz (bottom photo).

These photographs indicate that tin droplets having a diameter of about265 μm can be produced that are spaced apart by about 3.14 mm, a spacingwhich cannot be realized at this droplet size and repetition rate usinga single frequency, non-modulated waveform disturbance.

Measurements indicated a timing jitter of about 0.14% of a modulationperiod which is substantially less than the jitter observed undersimilar conditions using a single frequency, non-modulated waveformdisturbance. This effect is achieved because the individual dropletinstabilities are averaged over a number of coalescing droplets.

FIG. 7 shows a droplet pattern 600 produced using a modulated, e.g.,multiple frequency, disturbance waveform (see also FIG. 6, fourth photofrom top). Also shown, droplet pairs are formed at a selected distancefrom orifice 604. As shown, this droplet pattern in which droplet pairsreach the irradiation region allows droplet 608 a to establish an EUVemitting plasma upon irradiation by the laser 22′ while droplet 608 bshields subsequent droplet pair 610 from plasma debris.

FIG. 8 illustrates a droplet pattern 700 achievable using a modulatede.g., multiple frequency, disturbance waveform in which droplet pairsreach the irradiation region with a first droplet 702 a reflecting lightinto a self-directing laser system 704 to initiate a laser oscillationoutput laser beam which irradiates the second droplet 702 b to producean EUV emitting plasma.

Self-directing laser system 704 is more fully described in U.S. patentapplication Ser. No. 11/580,414, filed on Oct. 13, 2006, entitled, DRIVELASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE. See, in particular, FIG. 5of U.S. Ser. No. 11/580,414, the entire contents of which werepreviously incorporated by reference. Although the following describes alaser system 704 corresponding to FIG. 5 of the U.S. Ser. No.11/580,414, it is to be appreciated that this description is equallyapplicable to the other self-directed lasers disclosed in U.S. Ser. No.11/580,414 (i.e., FIGS. 6-16.)

Continuing with FIG. 8, it can be seen that the self-directing lasersystem 704 may include an optical amplifier 706 a,b,c. For example, theoptical amplifier 706 may be a CW pumped, multiple chamber, CO₂ laseramplifier amplifying light at a wavelength of 10.6 μm and having arelatively high two-pass gain (e.g., a two pass gain of about1,000,000). As further shown, the amplifier 706 may include a chain ofamplifier chambers 706 a-c, arranged in series, each chamber having itsown active media and excitation source, e.g., electrodes.

In use, the first droplet 702 a of target material is placed on atrajectory passing through or near a beam path 710 extending through theamplifier 706. Spontaneously emitted photons from the amplifier 706, maybe scattered by the droplet, and some scattered photons may be placed onpath 710 where they travel though the amplifier 706. As shown, an optic708 may be positioned to receive the photons on path 710 from theamplifier 706 and direct the beam back through the amplifier 706 forsubsequent interaction with the second droplet 702 b to produce an EUVlight emitting plasma. For this arrangement, the optic 708 may be, forexample, a flat mirror, curved mirror, phase-conjugate minor or cornerreflector. An optical element 714, e.g., lens may be positioned tocollimate light entering the amplifier 706 from the droplet and focuslight traveling from the amplifier 706 to the droplet. An optionaloptical delay 716 may be provided to establish the required time delaybetween when the first and second droplets reach the irradiation region.One advantage of using different droplets is that the size of thedroplets may be independently optimized for their specific function(i.e., reflection versus plasma production).

FIG. 8A illustrates a droplet pattern 700′ achievable using a modulatede.g., multiple frequency, disturbance waveform (described above), or apulsed waveform (described below), in which droplet doublets reach theirradiation site with each droplet in the droplet doublet beingirradiated to produce plasma. Specifically, both droplets in the doubletproduce plasma and EUV light, and both droplets may be irradiated by alaser beam produced with a single gain inversion in the amplifiers 706a′-c′. For this arrangement, an increased EUV output power may beobtained for each gain inversion relative to the EUV output producedwith a single droplet. In some cases, it may be advantageous to usedroplets, such that at least one droplet in the droplet doublet has adiameter, d, approaching the irradiation site, and center-to-centerdroplet spacing within each doublet, D, with d≦D≦4d. The droplets may besubstantially equal in diameter, or one droplet may be larger than theother.

Continuing with FIG. 8A, it can be seen that the self directing lasersystem 704′ may include optical amplifiers 706 a′,b′,c′. For example,the optical amplifier 706′ may be a CW pumped, multiple chamber, CO₂laser amplifier amplifying light at a wavelength of 10.6 μm and having arelatively high two-pass gain (e.g., a two-pass gain of about1,000,000). As further shown, the amplifier 706′ may include a chain ofamplifier chambers 706 a′-c′, arranged in series, each chamber havingits own active media and excitation source, e.g., electrodes.

In use, the first droplet 702 b′ of target material is placed on atrajectory passing through or near a beam path 710′ extending throughthe amplifier 706′. Spontaneously emitted photons from the amplifier706′ may be scattered by the droplet, and some scattered photons may beplaced on path 710′ where they travel though the amplifier 706′. Asshown, an optic 708′ may be positioned to receive the photons on path710′ from the amplifier 706′ and direct the beam back through theamplifier 706′. A laser beam may then be established along beam path710′ irradiating droplet 702 b′ and producing and EUV light emittingplasma and continue circulating in the optical cavity establishedbetween the plasma and optic 708′ until droplet 702 a′ reaches beam path710′. Droplet 702 a′ is then irradiated to produce EUV light emittingplasma. For this arrangement, the optic 708′ may be, for example, a flatmirror, curved mirror, phase-conjugate mirror or corner reflector. Anoptical element 714′, e.g., lens, may be positioned to collimate lightentering the amplifier 706′ from the droplet and focus light travelingfrom the amplifier 706′ to the droplet. In some implementations, thedroplets in the doublet may coalesce forming an elongated droplet priorto, or during, irradiation.

With reference now to FIGS. 9-12, Applicants have determined that inaddition to the modulated, e.g., multiple frequency, disturbancewaveforms described above, other waveforms may be used to producecoalescing droplet streams that can be controlled to produce a stablestream of coalesced droplets below the frequency minimum (about0.3ν/(πd) as described above), that would otherwise limit stable dropletproduction using single frequency sinusoidal non-modulated waveformdisturbances.

Specifically, these waveforms may produce a disturbance in the fluidwhich generates a stream of droplets having differing initial velocitieswithin the stream that are controlled, predicable, repeatable and/ornonrandom.

For example, for a droplet generator producing a disturbance using anelectro-actuable element, a series of pulse waveforms may be used witheach pulse having sufficiently short rise-time and/or fall-time comparedto the length of the waveform period to generate a fundamental frequencywithin an operable response range of the electro-actuatable element, andat least one harmonic of the fundamental frequency.

As used herein, the term fundamental frequency, and its derivatives andequivalents, means a frequency disturbing a fluid flowing to an outletorifice and/or a frequency applied to a sub-system generating droplets,such as a nozzle, having an electro-actuatable element producing adisturbance in the fluid; to produce a stream of droplets, such that ifthe droplets in the stream are allowed to fully coalesce into a patternof equally spaced droplets, there would be one fully coalesced dropletper period of the fundamental frequency.

Examples of suitable pulse waveforms include, but are not necessarilylimited to, a square wave (FIG. 9), rectangular wave, andpeaked-nonsinusoidal waves having sufficiently short rise-time and/orfall-time such as a fast pulse (FIG. 13A), fast ramp wave (FIG. 14A) anda sine function wave (FIG. 15A).

FIG. 9 shows a representation of a square wave 800 as a superposition ofodd harmonics of a sine wave signal. Note: only the first two harmonicsof the frequency f are shown for simplicity. It is to be appreciatedthat an exact square wave shape would be obtained with an infinitenumber of odd harmonics with progressively smaller amplitudes. In moredetail, a square wave 800 can be mathematically represented as acombination of sine waves with fundamental frequency, f, (waveform 802)of the square wave and its higher order odd harmonics, 3f, (waveform804), 5f (waveform 806); and so on:

${v(t)} = {\frac{4}{\pi}\left( {{\sin\left( {\omega\; t} \right)} + {\frac{1}{3}{\sin\left( {3\omega\; t} \right)}} + {\frac{1}{5}{\sin\left( {5\omega\; t} \right)}} + {\frac{1}{7}{\sin\left( {7\omega\; t} \right)}} + \ldots} \right)}$where t is time, v(t) is the instantaneous amplitude of the wave (i.e.voltage), and co is the angular frequency. Thus, applying a square wavesignal to an electro-actuatable element, e.g., piezoelectric, may resultin mechanical vibrations at the fundamental frequency f=ω/2π, as well ashigher harmonics of this frequency 3f, 5f, etc. This is possible due tothe limited and, in general case, highly nonuniform frequency responseof a droplet generator employing an electro-actuatable element. If thefundamental frequency of the square wave signal significantly exceedsthe limiting value of 0.3ν/(πd), then the formation of single dropletsat this frequency is effectively prohibited and the droplets aregenerated at the higher harmonics. As in the case of the amplitude andfrequency modulation described above, droplets produced with a squarewave signal have differential velocities, relative to adjacent dropletsin the stream, that lead to their eventual coalescence into largerdroplets with a frequency f. In some implementations, the EUV lightsource is configured such that a plurality of droplets are produced perperiod, with each droplet having a different initial velocity than asubsequent droplet, such that: 1) at least two droplets coalesce beforereaching the irradiation site; or 2) the droplets produce a desiredpattern such as a pattern which includes closely-spaced, dropletdoublets (see discussion below).

FIGS. 10 and 11 show images of droplets obtained with a square wavemodulation at 30 kHz. With a simple sine wave modulation, the lowestmodulation frequency where a single droplet per period can be obtainedfor the droplet generator used in this experiment was 110 kHz. The imageshown in FIG. 10 was taken at ˜40 mm from the output orifice and theimage shown in FIG. 11 was taken later at ˜120 mm from the outputorifice where the droplets are already coalesced. This exampledemonstrates the advantage of using a square wave modulation to obtaindroplets at a frequency lower than the natural, low-frequency limit of aparticular droplet generator configuration.

Similar arguments can be applied to a variety of repetitive modulationsignals with multiple harmonics having short rise-time and/or fall-timeincluding, but not limited to, a fast pulse (FIG. 13A), fast ramp wave(FIG. 14A) and a sine function wave (FIG. 15A). For instance, a sawtooth waveform contains not only odd, but also even harmonics of thefundamental frequency and therefore can also be effectively used forovercoming the low frequency modulation limit and improving stability ofa droplet generator. In some cases, a specific droplet generatorconfiguration may be more responsive to some frequencies than others. Inthis case, a waveform which generates a large number of frequencies ismore likely to include a frequency which matches the response frequencyof the particular droplet generator.

FIG. 12A shows a rectangular wave 900 for driving a droplet generatorand FIG. 12B shows a corresponding frequency spectrum having fundamentalfrequency 902 a and harmonics 902 b-h of various magnitudes for a periodof the rectangular wave. FIG. 12C shows an image of droplets taken at 20mm from the output orifice of the droplet generator driven by therectangular wave and shows droplets beginning to coalesce. FIG. 12Dshows an image of droplets taken at 450 mm from the output orifice afterthe droplets have fully coalesced.

FIG. 13A shows a series of fast pulses 1000 for driving a dropletgenerator and FIG. 13B shows a corresponding frequency spectrum havingfundamental frequency 1002 a and harmonics 1002 b-i of variousmagnitudes for a period of the rectangular wave. FIG. 13C shows an imageof droplets taken at 20 mm from the output orifice of the dropletgenerator driven by the series of fast pulses and shows dropletsbeginning to coalesce. FIG. 13D shows an image of droplets taken at 450mm from the output orifice after the droplets have fully coalesced.

FIG. 14A shows a fast ramp wave 1100 for driving a droplet generator andFIG. 14B shows a corresponding frequency spectrum having fundamentalfrequency 1102 a and harmonics 1102 b-p of various magnitudes for aperiod of the rectangular wave. FIG. 14C shows an image of dropletstaken at 20 mm from the output orifice of the droplet generator drivenby the fast ramp wave and shows droplets beginning to coalesce. FIG. 14Dshows an image of droplets taken at 450 mm from the output orifice afterthe droplets have fully coalesced.

FIG. 15A shows a sinc function wave 1200 for driving a droplet generatorand FIG. 15B shows a corresponding frequency spectrum having fundamentalfrequency 1202 a and harmonics 1202 b-l of various magnitudes for aperiod of the rectangular wave. FIG. 15C shows an image of dropletstaken at 20 mm from the output orifice of the droplet generator drivenby the sine function wave and shows droplets beginning to coalesce. FIG.15D shows an image of droplets taken at 450 mm from the output orificeafter the droplets have fully coalesced.

In accordance with one or more embodiments of the invention, it has beenreasoned by the inventors herein that one of the areas of improvement inefficiently and accurately coalescing droplets relates to theperformance of the bonding adhesive not at room temperature, i.e., thetemperature at which installation, maintenance, upgrade and/orinspection is performed, but at the higher operating temperature whenthe behavior of such adhesive is not readily observable. To elaborate,it is realized by the inventors herein that a seemingly secure adhesivebond at room temperature (the temperature at which installation,maintenance, inspection and/or upgrade is performed) would soften andtherefore negatively affect the transfer of acoustic/vibration energybetween the electro-actuable element (such as the piezoelectricmodulator in an embodiment) and the nozzle.

FIG. 16 shows a plot of the tensile modulus (Young's Modulus) of atypical high temperature epoxy adhesive (line 1602) with respect totemperature. As can be seen in FIG. 16, at room temperature of roughly23 degrees Celsius, the material has a tensile modulus of about 1.82GPa. The epoxy would appear solid and hard when thenozzle/electro-actuable element assembly is installed and/or inspectedat room temperature. At a temperature of about 250 degrees Celsius,which approximates the operating nozzle temperature in many EUV systems,the tensile modulus drops to 0.06 GPa. At this operating temperature,the epoxy stiffness would be reduced and would proximate that of a hardrubber material.

It is reasoned by the inventors herein that although not readilyobservable, this softening of the epoxy that bonds the electro-actuableelement to the nozzle detrimentally affects the ability of the acousticvibration waves to be efficiently transmitted to the nozzle forgenerating the desired disturbance with the desired degree of precision.Such softening would not be detectable in an obvious way since thenozzle operates at a vastly different temperature than the roomtemperature at which the nozzle is installed or inspected and isessentially inaccessible during EUV generation operation. As mentioned,epoxy that appears to be hard and appears to solidly bond theelectro-actuable element to the nozzle at room temperature would notgive motivation to select other exotic high temperature adhesiveswithout the aforementioned realization.

As is known to those skilled in the art, the group of resins intendedfor high temperature applications may include epoxy, polyester,vinylester, phenolic resins, cyanoacrylates, phenol-formaldehyde resins(i.e. Novolac), bismaleimides, and polyimides. In accordance with one ormore embodiments, polyimide-based adhesive is selected for use inbonding the electro-actuable element to the nozzle. Even thoughpolyimide-based adhesives are more expensive, more prone to moistureabsorption, and tend to have lower bond strength than epoxy adhesive,the aforementioned realization that epoxy, while seemingly solid at roomtemperature, presents a high impedance to acoustic/vibration waveformsat the typical nozzle operating temperatures motivates the inventorsherein to investigate polyimide-based adhesives for bonding theelectro-actuable element to the nozzle. Polyimide-based adhesives retainmuch of their stiffness at higher temperatures and are thus moreefficient at transmitting the acoustic/vibration energy from theelectro-actuable element to the glass nozzle at typical operatingtemperatures (in the hundreds of degrees Celsius).

In accordance with one or more embodiments, bismaleimide-based adhesiveis selected for use in bonding the electro-actuable element to thenozzle. Even though bismaleimide-based adhesives are more expensive,more prone to moisture absorption, and tend to have lower bond strengththan epoxy resin, the aforementioned realization that epoxy, whileseemingly solid at room temperature, presents a high impedance toacoustic/vibration waveforms at the typical nozzle operatingtemperatures motivates the inventors herein to investigatebismaleimide-based adhesive for bonding the electro-actuable element tothe nozzle. Bismaleimide-based adhesives retain much of their stiffnessat higher temperatures and are thus more efficient at transmitting theacoustic/vibration energy from the electro-actuable element to the glassnozzle at typical operating temperatures (in the hundreds of degreesCelsius).

Further improvement of the efficiency of transfer of acoustic vibrationsfrom the electro-actuable element to the nozzle can be achieved byincreasing modulus of the adhesive at operating temperature by way ofintroducing small particles of solid material to the adhesive. Forexample, microparticles of silver, silica, alumina, or another materialwith high modulus and with size significantly smaller than the gapbetween the electro-actuable element and the nozzle capillary can beused for this. In order to achieve an appreciable effect the combinedvolume of the added particles should be comparable to, or even greaterthan the volume of the adhesive, on the order of about 20 to about 90%of the total volume (total volume is the volume of the mixture that is acombination of added particles and resin), more preferably from about40% to 80% of the total volume and in a preferred embodiment, between50% to about 75% of the total volume.

Generally speaking, it is desirable to use an adhesive that has amodulus between 0.5 GPa (Giga Pascal) and 5 GPa at a nozzle operatingtemperature between 240 degrees Celsius and 270 degrees Celsius.

In accordance with one or more embodiments of the invention, it isrealized by the inventors herein that the frequency response of thenozzle assembly (which comprises at least the nozzle, theelectro-actuable element, and mechanism/arrangement that attaches theelectro-actuable element to the nozzle) depends on a variety of factors,including the construction, the mass, the shape of the components, etc.These different factors result in different resonance modes, wherein theresonances occur at different frequencies. However, it is theorized bythe inventors herein that if the resonance frequency of one of theresonance modes of the nozzle assembly can be made to coincide with themodulation frequency that is used to generate the disturbances, moreefficient perturbation of the nozzle can be achieved from the appliedmodulation signal.

In accordance with one or more embodiments of the invention, the nozzleassembly is characterized by plotting the nozzle assembly frequencyresponse versus frequency to ascertain the various resonance frequenciescorresponding to the different resonance modes of the nozzle assembly.More importantly, the frequency response is measured while the nozzleassembly is at its expected operating temperature (such as for exampleabout 250 degrees Celsius).

FIG. 17 shows a plot of such nozzle assembly frequency response. In FIG.17, both the amplitude (1702) and the phase (1704) of the nozzleassembly electrical impedance are plotted against frequency. Theimpedance phase plot line 1702 shows clearly multiple resonancefrequencies (shown by reference numbers 1712, 1714, 1716, for example).The lowest resonance frequency 1712 is shown to be about 250 kHz.

Thereafter, the nozzle assembly is modified such that one of theresonance to frequencies matches the modulation frequency. For example,for ease of modification, the nozzle assembly may be modified such thatthe lowest resonance frequency 1712 is shifted toward (and more closelymatches or ideally matches) the 40-80 KHz frequency of the modulationsignal from its current value of 250 kHz. However, there is efficiencybenefit by matching any resonance frequency (corresponding to the peaksof the phase shift plot for example) of the nozzle assembly with thefrequency of the modulation signal through nozzle assembly modification.

Modification of the nozzle assembly for resonance matching may include,for example, one or more of changing the mass of the nozzle tube,changing the mass of the electro-actuable element, changing the shape ofthe nozzle tube, changing the shape of the electro-actuable element,changing the material of the electro-actuable element or of the adhesiveemployed to attach the electro-actuable element to the glass nozzle.Modification of the nozzle assembly may also include for examplechanging the shape and/or construction of the electro-actuable element,the shape and/or construction of the nozzle tube, or the manner by whichthe piezoelectric transducer and the nozzle are assembled together.

FIG. 18A shows, in accordance with an embodiment of the invention, asideview of a nozzle assembly 1802 wherein the use of adhesive forcoupling the electro-actuable element 1804A/1804B to the nozzle 1806 iseliminated. Instead, a segmented electro-actuable element that includesa multi-piece (for example two-piece) electro-actuable element isemployed for coupling with the nozzle. In the example of FIG. 18A, atwo-piece electro-actuable element (such as piezoelectric modulator)comprising components 1804A and 1804B is shown. These components 1804Aand 1804B may have any external shape, including brick-shaped, cubicshaped, or any other arbitrary shape if desired. A groove is formed ineach component to accommodate nozzle 1806 such that the nozzle-facingsurface of the groove mates in an adhesive-less manner with thecomponent-facing surface of the nozzle to permit efficientacoustic/vibration energy transfer. A gap 1810 is shown disposed betweenadjacent components of the electro-actuable element to accommodatethermal expansion/contraction as the temperature of the nozzle assemblychanges. FIG. 18B is a view from direction 1820 of FIG. 18A to provideanother perspective of the adhesive-less implementation.

In one or more embodiments, the multi-piece electro-actuable element isbacked or surrounded (partially or wholly) by an appropriate enclosureor material that appropriately constrains the multi-pieceelectro-actuable element in place relative to the nozzle while stillpermitting some degree of movement for thermal expansion. By way ofexample, stiff resilient material or springs or adjustable screws/boltsor biasing members may substantially capture the multi-pieceelectro-actuable element in place and may apply a biasing force (shownin FIG. 18B) to force components of the multi-piece electro-actuableelement against the nozzle for efficient acoustic/vibration disturbancetransfer while still permitting some degree of movement to accommodatethermal expansion and contraction. In one or more embodiments, theenclosure and/or material backing or surrounding (partially or wholly)is chosen to have sufficient stiffness to reduce the displacement of themulti-piece electro-actuable element during operation while ensuringthat the acoustic/vibration energy produced by the multi-pieceelectro-actuable element is directed primarily or solely at the nozzleand not lost by electro-actuable element displacement.

The use of a multi-piece electro-actuable element to surround anddirectly couple with the nozzle outer surface is desirable since theelectro-actuable element and the nozzle typically have different thermalexpansion rates. If the electro-actuable element had been a single pieceand had simply enclosed the outer periphery of the glass nozzle, damageto the glass nozzle and/or the electro-actuable element may result dueto thermal stress.

While the particular embodiment(s) described and illustrated in thisPatent Application in the detail required to satisfy 35 U.S.C. §112 arefully capable of attaining one or more of the above-described purposesfor, problems to be solved by, or any other reasons for, or objects ofthe embodiment(s) above-described, it is to be understood by thoseskilled in the art that the above-described embodiment(s) are merelyexemplary, illustrative and representative of the subject matter whichis broadly contemplated by the present application. Reference to anelement in the following Claims in the singular, is not intended to meannor shall it mean in interpreting such Claim element “one and only one”unless explicitly so stated, but rather “one or more”. All structuraland functional equivalents to any of the elements of the above-describedembodiment(s) that are known, or later come to be known to those ofordinary skill in the art, are expressly incorporated herein byreference and are intended to be encompassed by the present Claims. Anyterm used in the Specification and/or in the Claims and expressly givena meaning in the Specification and/or Claims in the present Applicationshall have that meaning, regardless of any dictionary or other commonlyused meaning for such a term. It is not intended or necessary for adevice or method discussed in the Specification as an embodiment toaddress or solve each and every problem discussed in this Applicationfor it to be encompassed by the present Claims. No element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the Claims. No claim element in the appendedClaims is to be construed under the provisions of 35 U.S.C. §112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or, in the case of a method claim, the element is recited asa “step” instead of an “act”.

What is claimed is:
 1. A plasma generating system comprising a source oftarget material droplets and a laser producing a beam irradiating thedroplets at an irradiation region, the plasma producing EUV radiation,wherein the droplet source comprises a nozzle having an orificeconfigured for ejecting a fluid and a sub-system having anelectro-actuable element producing a disturbance in the fluid to causeat least some of the droplets to coalesce prior to being irradiated, theelectro-actuable element being coupled to nozzle using an adhesivehaving a modulus between 0.5 GPa and 5 GPa at a nozzle operatingtemperature between 240 degrees Celsius and 270 degrees Celsius.
 2. Theplasma generating system of claim 1 wherein said electro-actuableelement is a piezoelectric modulator.
 3. The plasma generating system ofclaim 1 wherein the disturbance comprises a frequency-modulatedwaveform.
 4. The plasma generating system of claim 1 wherein thedisturbance comprises an amplitude-modulated waveform.
 5. The plasmagenerating system of claim 1 wherein the disturbance comprises a seriesof pulsed disturbances, with each pulsed disturbance of the series ofpulsed disturbances having at least one of a sufficiently shortrise-time and sufficiently short fall-time to generate a fundamentalfrequency and at least one harmonic of the fundamental frequency.
 6. Theplasma generating system of claim 1 wherein said adhesive ispolyimide-based.
 7. The plasma generating system of claim 1 wherein saidadhesive is bismaleimide-based.
 8. The plasma generating system of claim1 wherein said adhesive includes a mixture of resin and particles ofsolid material.
 9. The plasma generating system of claim 8 wherein saidparticles of solid material represent microparticles of one of silver,silica, or alumina.
 10. The plasma generating system of claim 8 whereinsaid particles of solid material comprise about 20% to 90% of the totalvolume.